1. Field
The present disclosure relates to the field of olefin polymerization. It more particularly relates to methods and designs of preventing fouling in olefin polymerization reactors. Still more particularly, the present disclosure relates to methods and designs of preventing fouling in olefin polymerization reactors when the reactor is operating in supercritical or liquid-filled solution polymerization modes.
2. Description of the Related Art
International Patent Publication No. WO 2006/044149, herein incorporated by reference in its entirety, discloses a polymerization process where the polymer dissolution is substantially aided by an inert solvent (also referred to herein as “solution process”), such as where the polymerization medium contains more than 20 wt % solvent, and typically more than 60 wt % solvent. When the reactor pressure in a solution polymerization process is above the bubble point pressure of the liquid polymerization system, the entire reactor is filled with liquid without a vapor phase being present. Such operation mode of a solution polymerization process will be referred to herein as liquid-filled reactor operation. Commercial solution polymerization reactors often operate in this liquid-filled operation regime for maximum utilization of the entire reactor volume.
U.S. Patent Publication Nos. 2006-0211832 and 2006-0025545, herein incorporated by reference in their entirety, disclose a flexible homogeneous polymerization platform for the homogeneous supercritical polymerization of propylene-containing monomer feeds (also referred to herein as the “supercritical process”, or “supercritical polymerization”, or “supercritical polymerization process”, or “supercritical propylene polymerization”). In the referred supercritical polymerization process, polymerization is often carried out in a substantially bulk supercritical monomer medium, thus it is a bulk homogeneous supercritical polymerization process. The polymeric products in these supercritical processes are in a homogeneously dissolved state in the reactor. Since supercritical fluids completely fill the volume available to them, the reactors of said supercritical polymerization processes are also completely filled with the supercritical fluid polymerization system, and thus in this regard are similar to the liquid-filled solution polymerization process. However, the supercritical polymerization processes provide an advantageous means to the so-called solution processes in their ability to flexibly and robustly both produce highly crystalline, high molecular weight (i.e. low melt-flow rate) isotactic propylene homopolymers and low-crystallinity and/or low molecular weight homo- and copolymers.
Both solution and homogeneous supercritical polymerization processes may be performed in a bulk monomer phase using essentially pure monomer(s) as solvent or may keep the polymer in the homogeneous fluid state by employing an inert solvent in substantial concentrations (i.e., 40 wt % or more or 60 wt % or more). The solution process provides for a polymer-containing liquid phase either in an inert solvent or in the essentially neat monomer or in their mixture in their liquid state. The homogeneous supercritical process provides for the polymeric fluid state by dissolving the polymeric product either in an inert solvent or in the essentially neat monomer or in their mixture in their supercritical state. When solution polymerization is performed in the liquid-filled mode, i.e., at reactor pressures above the bubble point of the liquid-phase polymerization system, it shares a common characteristics with the supercritical polymerization process in that the polymerization system in both processes fills the entire reactor volume. An important consequence of this feature is that there are no parts of the reaction zone within the interior of the reactor that are not in contact with the dissolved polymeric product.
As stated above, both the solution and the homogeneous supercritical polymerization processes provide polymers dissolved in a homogeneous fluid state where the polymer-containing fluid fills the entire volume of the reactor. Because in both solution and supercritical polymerization processes the polymer-containing reaction mixture fills the entire volume of the polymerization reactor, the polymer may get into contact with cold parts of the reactor, may crystallize and thus cause fouling at those cold spots in the interior reactor volume, and/or in its interconnected parts, such as stirrers of stirred reactors, or feed or recycle ports. Fouling in turn results in serious operational issues in terms of costly reactor downtime required for cleaning that correspondingly decreases production efficiencies. Hence, there is a need for improved reactor designs and methods of operating reactors when polymerizing polymers via either the solution process or supercritical process to reduce the propensity for fouling of the interior reactor volume. The methods disclosed herein offer reduced fouling and fouling-free stirrer and feed port designs and operation conditions for homogeneous supercritical and solution polymerization processes.